1. Field of the Invention
The present invention relates to an erasable ball-point pen ink. More specifically, the invention relates to an ink that is particularly adapted to be used with ball-point pens for writing fine letters which leaves ink traces that remain erasable with a rubber eraser for a certain period of time after writing.
2. Description of the Prior Art
Prior art techniques concerning inks of the type contemplated by the present invention are disclosed in Japanese Unexamined Published Patent Application Nos. 156731/79, 155266/81, 170967/82 and 170968/82.
The inks proposed in these patent applications have two things in common. First, in order to maintain the erasability of ink traces, pigments, not dyes, are used as ink colorants, and they are combined with rubbers to provide ink suspensions having very high viscosities (1 to 6.times.10.sup.6 cps). Second, the rubbers are dissolved in volatile solvents, which, after writing, rapidly evaporate from the ink, thus increasing the viscosity of the ink traces and preventing penetration of the ink into the paper. As a natural consequence of this mechanism of ink erasability, the small clearance defined by the writing ball and the ball holder has a tendency to be clogged by agglomerates in the ink while the ink is passing through the clearance during writing. If the writing tip is left uncapped for several months, the high viscosity of the ink, coupled with the evaporation of the volatile solvent that occurs at the writing tip, upsets the stability of the pigment suspension and increases the chance of formation of agglomerates in the ink. Additionally, the inks disclosed in the above listed prior art references ensure smooth ink flow from the writing tip by using a lubricant selected from higher aliphatic acids such as stearic acid, palmitic acid, oleic acid and lauric acid. However, these higher aliphatic acids greatly increase the chance of ink clogging at the writing tip if the tip is left uncapped in a humid atmosphere. A probable reason for this phenomenon is as follows: the carboxyl group in an aliphatic acid exhibits such strong hydrophilicity that water is adsorbed onto the writing tip to upset the uniformity of the composition of the oily ink suspension, and the resulting agglomerates in the suspension, coupled with the increase in viscosity due to the evaporation of the volatile solvent, cause plugging of the writing tip.
With ball-point pens intended for writing thick letters with a ball diameter of 1.0 mm, clogging of the writing tip seldom occurs, but this is not the case for fine letter writing ball-point pens with a ball diameter of 0.7 to 0.8 mm (see Comparative Examples 1 to 3 later presented). Presently, no fine letter writing ball-point pen is available that ensures a very smooth ink flow and letter writing during any season of the year without blocking of the small clearance between the writing ball and the ball holder even when the writing tip is left uncapped and exposed to the atmosphere for a prolonged period.